Hydrocarbon drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information may include characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is commonly termed “logging.”
Drillers often simultaneously log a borehole while drilling, and thereby eliminate the need of removing or “tripping” the drilling assembly to insert a wireline logging tool to collect the required data. Data collection during drilling also enables the driller to make accurate modifications or corrections as needed to steer the well or optimize drilling performance while minimizing down time. Designs for measuring conditions downhole including the movement and location of the drilling assembly contemporaneously with the drilling of the well have come to be known as “measurement-while-drilling” techniques, or “MWD.” Similar techniques that concentrate more on the measurement of formation parameters are commonly referred to as “logging-while-drilling” techniques, or “LWD.” While distinctions between MWD and LWD may exist, the terms MWD and LWD are often used interchangeably.
In MWD and LWD tools, sensors in the drill string measure the desired drilling parameters and formation characteristics and continuously or intermittently transmit the information obtained to a surface detector by some form of telemetry. Most MWD and LWD tools use the drilling fluid (or mud) circulating through the drill string as the information carrier, and are thus referred to as mud pulse telemetry systems. In positive-pulse systems, a valve or other form of flow restrictor creates pressure pulses in the fluid flow by adjusting the size of a constriction inside the drill string. In negative-pulse systems, a valve creates pressure pulses by releasing fluid from the interior of the drill string into the annulus surrounding the drill string. In both system types, the pressure pulses propagate at the speed of sound through the drilling fluid to the surface, where they are detected by various types of surface transducers.
Drilling operations have become more complicated and customers are requiring more downhole sensors. This means that more data is required to be transmitted uphole in the same period of time, and thus higher data rates are now needed. At the same time, wells are getting deeper and directional wells are getting longer, which leads to the MWD and LWD tools being required to operate reliably for longer periods of time. Increasing the usable life of the MWD and LWD tools is a useful aspect in providing a competitive advantage in the marketplace.